The present invention relates to power amplifiers and, in particular, to a base drive circuit for increasing the speed and efficiency of a flyback output transistor.
Power amplifiers designed to function as flyback output circuits typically operate as high current switching devices that drive inductive loads, such as a yoke mechanism of a horizontal deflection circuit of a display system employing a cathode ray tube. Such an amplifier is described in U.S. Pat. No. 4,670,692 by Meigs and assigned to Tektronix, Inc. Meigs describes a high speed power amplifier for driving inductive loads at frequencies of at least 270 khz. A continuous square-wave signal having a 50% duty cycle is applied to the base of an output flyback transistor through an inductor and a resistor. On the first positive cycle of the square-wave signal, current flows through the inductor and the resistor and into the base of the output transistor. On the downward cycle of the square-wave signal, a shunt diode, in parallel with the inductor and the resistor, is biased into conduction to remove an induced storage charge in the base of the output transistor.
A significant drawback to this type of base drive is that the stored flux in the inductor is not totally dissipated during the removal of the induced storage charge on the base of the transistor. This causes the current flowing into the base of the transistor to step-up sharply at the start of the next positive cycle of the square-wave signal. The current flowing into the base gradually increases from the initial sharp step until the next downward cycle of the square-wave signal. The initial sharp increase in the base drive current decreases the efficiency of the output transistor and induces excess storage charge.
An additional drawback to the Meigs' design is the time required to remove the induced storage charge in the base of the output transistor. Approximately one microsecond elapses between the start of the reverse current and the time the current reaches a maximum. This response time limits the operating speed of the output transistor.
What is needed is a base drive circuit that linearly increases the base drive current to an output transistor from an initial zero value. In addition, to increase the speed of the output transistor the reverse base current needs to be very high and turn on very rapidly.